1. Field of the Invention
This invention relates generally to novel methods for producing lightweight, foams and, in particular, to methods for producing foams using melt processing techniques and blowing agents combining physical and chemical blowing agents to enable secondary expansion of the foam.
2. Description of the Background
Polymeric foams include a plurality of voids, also called cells, in a polymer matrix. By replacing solid plastic with voids, polymeric foams use fewer raw materials than solid plastics for a given volume. Thus, by using polymeric foams instead of solid plastics, material costs can be reduced in many applications. Additionally, foams are very good insulators that can seal building structures from air and moisture intrusion, save on utility bills, and add strength to the building.
The material used for expandable polystyrene (EPS) is typically an amorphous polymer that exhibits a glass transition temperature of about 95° C. The process of converting EPS resins into expanded polystyrene foam articles requires three main stages: pre-expansion, maturation, and molding. Expandable beads produced from polystyrene and a blowing agent are made, and then expanded by steam in a pre-expander. The purpose of pre-expansion is to produce foam particles of the desired density for a specific application. During pre-expansion, the EPS beads are fed to a pre-expander vessel containing an agitator and controlled steam and air supplies. The introduction of steam into the pre-expander yields two effects: the EPS beads soften and the blowing agent that is dispersed within the EPS beads, typically pentane, heats to a temperature above its boiling point. These two conditions cause the EPS beads to expand in volume. The diameter of the particles increases while the density of the resin decreases. The density of pre-expanded granules is about 1000 kg/m3, and that of expanded beads lies in the range of 20 to 200 kg/m3; depending on the process, a 5 to 50 times reduction in density may be achieved.
Maturation serves several purposes. It allows the vacuum that was created within the cells of the foam particles during pre-expansion to reach equilibrium with the surrounding atmospheric pressure. It permits residual moisture on the surface of the foam particles to evaporate. And, it provides for the dissipation of excess residual blowing agent. Maturation time depends on numerous factors, including blowing agent content of the original resin, pre-expanded density, and environmental factors. Pre-expanded beads that are not properly matured are sensitive to physical and thermal shock. Molding of such beads before maturation may cause the cells within the particles to rupture, thereby producing an undesirable molded foam part.
Once the pre-expanded beads have matured, they are transferred to a molding machine containing one or more cavities that are shaped for the desired molded foam article(s). The purpose of molding is to fuse the foam particles together into a single foam part. Molding of EPS may follow a simple sequence: first, fill the mold cavity with pre-expanded beads; heat the mold by introducing steam; cool the molded foam article within the mold cavity; and eject the finished part from the mold cavity. The steam that is introduced to the molding machine causes the beads to soften and expand even further due to release of residual blowing agent, such as pentane. The combination of these two effects in an enclosed cavity allows the individual particles to fuse together into a single solid foam part.
There is an increasing demand for many plastic products used in packaging to be biodegradable, for example trays in cookie and candy packages. Starch films have been proposed as biodegradable alternatives for some time. A common approach to creating biodegradable products is to combine polylactic acid (PLA) with starch to create a hydrolytically degradable composition. Difficulties have been encountered in producing starch based polymers particularly by hot melt extrusion. The molecular structure of the starch is adversely affected by the shear stresses and temperature conditions needed to plasticize the starch and pass it through an extrusion die.
Blowing agents typically are introduced into polymeric material to make polymer foams in one of two ways. According to one technique, a chemical blowing agent is mixed with the polymer. The chemical blowing agent undergoes a chemical reaction in the polymeric material, typically under conditions in which the polymer is molten, causing formation of a gas. Chemical blowing agents generally are low molecular weight organic compounds that decompose at a particular temperature and release a gas such as nitrogen, carbon dioxide, or carbon monoxide. According to another technique, a physical blowing agent, i.e., a fluid that is a gas under ambient conditions, is injected into a molten polymeric stream to form a mixture. The mixture is subjected to a pressure drop, causing the blowing agent to expand and form bubbles (cells) in the polymer. Several patents and patent publications describe aspects of microcellular materials and microcellular processes.
U.S. Pat. No. 6,593,384 to Anderson et al. describes expandable particles produced using broad polymer materials and a physical blowing agent. U.S. Pat. No. 7,226,615 to Yuksel et al. describes an expandable foam based on broad disclosure of biomaterials combined with a bicarbonate blowing agent. U.S. Published Patent Application No. 2006/0167122 by Haraguchi et al. describes expandable particles derived from the combination of PLA, a blowing agent, and a polyolefin wax. U.S. Published Patent Application No. 2010/0029793 by Witt et al. describes a method of producing PLA foam by impregnating resin beads with carbon dioxide (CO2).
U.S. Pat. No. 4,473,665 to Martini-Vvedensky et al. describes a process for making a foamed polymer having cells less than about 100 microns in diameter. In the described technique, a material precursor is saturated with a blowing agent, the material is placed under high pressure, and the pressure is rapidly dropped to nucleate the blowing agent and to allow the formation of cells. The material then is frozen rapidly to maintain a desired distribution of microcells.
U.S. Pat. No. 5,158,986 to Cha et al. describes formation of microcellular polymeric material using a supercritical fluid as a blowing agent. Using a batch process, the patent describes various processes to create nucleation sites.
U.S. Pat. No. 5,866,053 to Park et al. describes a continuous process for forming microcellular foam. The pressure on a single-phase solution of blowing agent and polymer is rapidly dropped to nucleate the material. The nucleation rate is high enough to form a microcellular structure in the final product.
International patent publication no. WO 98/08667 by Burnham et al. provides methods and systems for producing microcellular material and microcellular articles. In one method, a fluid, single-phase solution of a precursor of foamed polymeric material and a blowing agent is continuously nucleated by dividing the stream into separate portions and separately nucleating each of the separate portions, then recombining the streams. The recombined stream may be shaped into a desired form, for example by a shaping die.
It is generally accepted in the field that to create enough nucleation sites to form microcellular foams, one must use sufficient blowing agent to create a driving force for nucleation and a high enough pressure drop rate to prevent cell growth from dominating the nucleation event. As blowing agent levels are lowered, the driving force for nucleation decreases. Yet, while higher blowing agent levels can lead to smaller cells (a generally desirable result in the field of microcellular foams), according to conventional thought, higher blowing agent levels also can cause cell interconnection (which by definition increases cell size and can compromise structural and other material properties) and less-than-optimal surface properties (compromised surface properties at higher gas levels can result from the natural tendency of the blowing agent to diffuse out of the material).
In other words, it is generally accepted that there is a tradeoff between small cell size and optimal material properties as blowing agent levels in microcellular polymeric material are altered.